The present invention relates to charged particle beam apparatuses such as electron microscopes and focused ion beam systems.
A conventional scanning electron microscope (SEM) creates an image as follows: an electron beam emitted from an electron gun consisting of a field emitter or thermal field emitter electron source is accelerated and made a thin electron beam by an electron lens; this beam as a primary electron beam is scanned over a sample through a scanning deflector and generated secondary electrons or backscattered electrons are detected. The material of the electron source for a general-purpose SEM is tungsten. The material of the electron source for semiconductor observation may be tungsten which contains zirconia.
To ensure that the electron source emits a good quality electron beam for a long time, the area around the electron source must be maintained in a high vacuum condition (10−7 to 10−8 Pa). For this purpose, as shown in FIG. 8, conventionally a plurality of ion pumps (three pumps IP-1, IP-2, IP-3 in the figure) has been used to evacuate the column by differential pumping. This method is disclosed in Japanese Patent Application Laid-Open No. 2002-358920. Although an ion pump has an advantageous that it has no movable part and can maintain the pressure 10−8 Pa or less simply by supplying power to it, its size is several dozen centimeters square or more and a magnetic shield is needed for the column (because it generates a magnetic field) and therefore its space requirement and weight are considerable.
One approach to a compact apparatus without ion pumps is disclosed in U.S. Pat. No. 4,833,362 and Japanese Patent Application Laid-Open No. 6-111745 where a non-evaporable getter pump is incorporated. As another approach, Japanese Patent Application Laid-Open No. 2000-149850 describes an electron gun which incorporates a getter ion pump, eliminating the need for conventional ion pumps.
A further approach described in Japanese Patent Application Laid-Open No. 2004-202309 is a non-evaporable getter pump in which non-evaporable getter alloy is placed in a space enclosed by a mesh in order to prevent generation of foreign particles and the mesh is finer than microparticles which are generated.